1. Field of the Invention
This invention relates generally to digital data transmission systems. More particularly, this invention relates to an apparatus and method for adaptive equalization to be utilized in a digital subscriber loop for inter-symbol interference (ISI) cancellation.
2. Description of the Prior Art
The increased demand for data communication including the need to transmit digitized audio and video data at a very high baud rate among user sites has placed great burden on the existing communication networks. The narrow-band telephone lines could potentially be utilized for transmission of these digital data among all user sites. However, the use of the existing user subscribed loops is limited by the inter symbol interference (ISI), especially for high baudrate transmissions of these digitized data.
The interface circuit in a digital user subscribed loop usually comprises a transmitter and a receiver. In the Basic Rate Application of the Integrated Service Digital Network (ISDN) system, the U-interface transceiver transmits and receives digital data at a rate of 160 Kb/s. The data is formatted into frames of 240 bits or 120 bauds including nine baud Barker code for frame synchronization and then encoded by a scrambler to become pseudo-random bit stream. The bit stream is then converted into two-binary, one quaternary (2B1Q) code as proposed by the American National Standard Institute (ANSI) Working Group T1E1. The 2B1Q encoder makes the combination of binary bits 00, 01, 11, and 10 mapped into four corresponding `symbol` level of -3, -1, +1, and +3 respectively. The transmitter filter transfers the symbol levels into analog pulses before the signals are transmitted over the subscriber loops to the remote site.
The input signals to the U-interface transceiver are sampled at a sampling rate of 80 Kbaud per second. These samples are used by the receiver to deride the original digital data via some digital signal process. A typical impulse response of the subscriber loop is shown in FIG. 1. The impulse response is sampled at receiver which generates a plurality of discrete channel responses, i.e., channel responses 1 to 8 as denoted as h.sub.0, h.sub.1, . . . , h.sub.6, h.sub.-1, h.sub.-2, respectively. The main pulse is identified with a numeral reference 1, i.e., h.sub.0, wherein the peak of the pulse is generally referred to as a cursor. In addition to the main pulse 1, there are a pre-cursor under shot 9, i.e., h.sub.-2 and a long gently decreasing post-cursor tail as represented by channel responses h.sub.3,h.sub.4,h.sub.5, and h.sub.6, which may extend to overlap the next main pulse thus creating the `inter-symbol` interference (ISI). The sampling timing t.sub.0 is arranged such that a measurement of h.sub.0 is substantially coincide with the peak of the pulse such that a maximum value of h.sub.0 is obtained.
Since these pulses are distorted in the process of transmission, i.e., the transmitted pulses are smeared, direct detection of these pulses which is required to be carried out by the receiver thus becomes quite difficult. There are several sources of signal impairment which introduce different types of distortions into the signal pulses in the process of transmission. The first type is caused by the coupling between pulses across the hybrid circuits. This is a common problem when the signals are transmitted over a two-wire system. Such type of distortion is generally referred to as echoes. A transversal filter which utilizes an echo-cancellation algorithm is usually used by a receiver to remove this type of signal distortions.
Other than the echoes, there are also signals distortions caused by inter-symbol interferences (ISI). As shown in FIG. 1, the tail of the distorted impulse response of one pulse may extend to several following pulses. The inter-symbol interference (ISI) is thus generated due to the facts that in sampling a transmitted signal, a receiver received not only the desired signal of the current `symbol, i.e. S(n), the sampling results comprises signal components S(n+i) due to the precursor tail, and the signal component S(n-i) due to the post cursor tail, where i denotes an integer. The ISI distortions are usually dealt with by means of a decision feedback equalizer (DFE) with a feedforward filter.
In U.S. Pat. No. 4,995,031 entitled `Equalizer for ISDN-U Interface` issued on Feb. 19, 1991, Aly et al. disclose a receiver for a digital data transmission system which comprises a pre-cursor equalizer. The precursor equalizer utilizes a difference equation such that the equalized digital signals have at least one zero crossing occurring substantially one baud before the main cursor of each pulse.
Although the proposed equalizer by Aly et al. is very effective to reduce the pre-cursor ISI distortions of the received signals, the error propagation caused by the strong post-cursor ISI distortions introduced through the feedback path is not properly processed which may cause a prolonged convergence time thus substantially slowing down the system response. Furthermore, the implementation requires very precise timing circuit and highly sophisticated digital signal processor (DSP) in order to accurately control the sampling time and to assure that the zero crossings of the precursor undershoot occur at an integer multipliers of the sampling period. Such implementation can be very costly and may not be economically feasible in the near future.
Kuenast in another U.S. Pat. No. 5,027,369 entitled `Rapid Convergence Decision Feedback Equalizer` issued on Mar. 26, 1990, also discloses a DFE which has two separate portions each functions as an individual DFE having different number of taps and different adaptive tap sizes to cancel the post cursor distortions. The two stage DFE equalizer changes the length of the feedback path to prevent error propagating and shorten the training period of the equalizer. However, the proposed equalizer can be quite complicate in design and the coordination between the first and second DFE is also not well defined to assure rapid convergence can be achieved. Additionally, the error propagation may still hinder the efficiency of convergence during the initially transient stage in DFE's process without taking into account the correlation between the precursor and the post-cursor distortions.
Crespo et al. discloses in U.S. Pat. No. 5,031,194 entitled `Wideband Digital Equalizer for Subscriber Loops` a decision feedback equalizer which compensates the post-cursor ISI by dividing the impulse response of the transmission channel into two regions. A fast acting, close-tracking linear filter is used to compensate the initial rapidly changing unpredictable responses while a slow acting simple, simple, pole-zero filter is used to compensate a relative slowly changing, more predictable asymptotic tail of the impulse response. Again, this may have the same difficulties and limitations as encountered by Kuenast as discussed above. Furthermore, a system stability problem may occur by the use of the infinite impulse response (IIR) filter as disclosed by Crespo et al. which also adds to the complexity of applying the method for ISI cancellations.
Therefore, for those skilled in the art, a need still exists for an improved equalization system and method which is simple in design, easy to be implemented and yet effective and efficient in reducing the transmission distortions including the ISIs from the received signals transmitted over a digital subscriber loop.